Document yVgqjE04onQ1y9pYrDxRwgnn

266 CHAPTER IS 1948 Guide Table 9. Relation Between Local Mean Sun Time and Solar Altitude During the Period May 2-August 10 for North Latitudes Local Mean Sun Time A.M. P.M. 5 6 7 8 9 10 11 12 7 6 5 4 3 2 1 . 25 0 8.5 21.5 35. 4628..5 75.5 85. Solar Altitude North Latitude--Degrees 30 35 40 45 - 0 10. 0 11. 2. 4. 12.5 14. 22.5 34.5 . 48. 61. 73. 80. 23.5 35.5 47.5 59.5 70. 75. 24. 35.5 47. 24.5 35. 45.5 6567..5 55. 62. 70. 65. 50 6. 15. 24.5 34.5 43.5 51.5 60. outdoor air which, in contact, with the weather side of a material that is receiving no solar or sky radiation, would give the same rate of heat entry into that surface as would exist with the actual combination of incident solar and sky radiation and convective heat transfer. ' In order to calculate the rate of heat transfer into an outside building* surface for any instant of time, it is necessary to consider: 1. The intensity of direct solar radiation striking the surface. 2. The absorptivity of the surface for direct solar radiation. 3. The intensity of sky radiation striking the surface. 4. The absorptivity of the surface for sky radiation. 5. The rate at which the surface emits radiation to the sky. 6. The temperature of the surrounding air. 7. The temperature of the outer building surface. 8. The unit convective conductance for heat transfer between the air and the building surface. The magnitude of the convective conductance depends upon the position of the surface and the velocity of the wind or air currents. The simultaneous consideration of all these effects is too complex for practical application and therefore the sol-air temperature is developed as follows: 1.The equation for the rate of heat transfer into the weather side of a sunlit building material at any instant is written: = bit + fo (k> -- <l) Btu per (hour) (square foot). (4) where^b = Absorptivity of weather side of material for incident solar and sky radiation, dimensionless. It = Rate of incidence of solar and sky radiation, Btu per (hour) (square foot). to = Outside air temperature, Fahrenheit degrees. tL = Temperature of weather surface of the material, Fahrenheit degrees. fo *= Unit convective conductance, Btu per (hour) (square foot) (Fahrenheit degree). 2.The sol-air temperature is defined as: bit le = to + fo (5) 3. Then, the instantaneous rate of heat entry into the weather side of the structure becomes: = fo (*e -- tL) Btu per (hour) (square foot) (6) Example 1. If to = 90 F, b = 0.7,1 -- 200 Btu per.(hour) (square foot), and/0 = 4, find the sol-air temperature, te. Substituting these values in Equation 5: Cooling Load 267 90 + 0.7 (200) = 125 F. Thus the instantaneous rate of entry of heat into the weather side of this material is precisely the same as if the air temperature were 125 F with no solar and sky radiation exchange with the surface. The preceding sol-air concept is intended for non-glass building areas. For glass surfaces a similar method could be developed, with the addition of terms to account for absorption within the material and outward radiation from the interior space; It is customary, however, to treat glass separately; see section on Glass Areas in this chapter. The sol-air temperature is a composite quantity the magnitude of which is influenced by each of the variables entering its defining equation. Establishing its magnitude for design calculations is part of the broad problem of determining weather-design data* Sufficient studies have been completed, however, to produce some sol-air data of practical value. Data of the U. S. Weather Bureau for the 10-year period from 1932 through 1941 have been studied for New York, N. Y.,15 a2n3d*Lincoln, Nebr. 6 Only simultaneous values of the air temperature and solar and sky radiation have been combined in determining design values of the solair temperature for various surfaces at different times of day in these localities. Since the 24-hour average of the sol-air temperature is greater in July than for any other month at both stations, the sol-air temperature at each hour in July, which was equalled or exceeded at that hour only 16 times in 310 observations, was chosen as the design sol-air temperature. Summer design sol-air temperatures are given in Table 10 for New York, N. Y.; Table 11 gives similar data for Lincoln, Nebr. These data may be taken as representative for similar places in northern latitudes in the United States. Tables 10 and 11, referring to New York for an industrial area and to Lincoln, Nebr., for a non-industrial area, can be used to determine sol-air temperatures for other locations and conditions by applying corrections as explained in the following paragraphs (using the same symbols as given' in Tables 10 and 11 but indicated by (') for other conditions): 1. To adjust the data in Tables 10 and 11 for the variation of It with latitude: a. Compute I\ for the latitude in question from the data presented previously in this chapter. (For smoky industrial areas, decrease the direct radiation from Table 5 by 15 to 20 per cent for afternoon hours.) b. Determine the difference (/'t -- It) and multiply this by 0.25. Find It = ifo/b) (to -- to) from Tables 10 and 11. c. Add (algebraically) the difference 0.25 (/'t -- It) to the data tabulated. 2. To adjust the data in Tables 10 and 11 for variations in to'. a. Establish the magnitude of t'0 at the locality in question. b. Determine the difference (t'o -- to)c. Add (algebraically) the difference (t'o -- t0) to the data tabulated. 3. To adjust the data in Tables 10 and 11 to other,magnitudes of 6//0 than 0.25: a. For New York or Lincoln, merely interpolate or extrapolate the tabulated data by direct proportion, using the column for b/f0 = 0. b. For other localities, first determine t'o = t'o + b/fa I't for b/f0 = 0 and b/fo = 0.25. Then interpolate or extrapolate by direct proportion as before. Sol-air data for a particular locality may be adjusted for different azimuths of the vertical surface in question in a similar manner; although,